1. Field of the Invention
The present invention relates to driving devices, particularly a driving device for providing light dimming control of a light-emitting element.
2. Description of the Background Art
Illumination devices such as incandescent lamps and streetlight employing light-emitting elements such as an LED (Light Emitting Diode) are developed.
As a method of adjusting the light of a light-emitting element, PWM (Pulse Width Modulation) control of the luminance of an LED is known. This method is directed to adjusting the luminance of an LED by altering the pulse width (time width) of the current flowing through the LED, i.e. by altering the duty of the pulse of the current flowing through the LED.
For example, Japanese Patent Laying-Open No. 2006-324671 (Patent Document 1) discloses an LED driving circuit set forth below. The LED driving circuit includes a PWM driving unit providing a switching pulse having the width adjusted by the voltage detected from a voltage detection resistor to a switch to control the ON/OFF time of the switch, a comparison unit comparing the voltage value detected by the voltage detection resistor and an arbitrary dimming voltage value, and a PWM control unit generating a control signal to increase and reduce the ON time of the switching pulse output from the PWM driving unit when the comparison voltage value from the comparison unit is increased and decreased, respectively. The luminance of the LED is adjusted by controlling the switching pulse width output from the PWM driving unit according to the dimming voltage value.
In the LED driving circuit of Patent Document 1, the current flowing through the LED will not be constant even in the case where the dimming voltage is maintained constant if the properties such as the LED forward voltage and color temperature vary. Therefore, this LED driving circuit is disadvantageous in that the luminance and chromaticity will vary.